Recently, with the small-sizing or miniaturizing tendency of electric parts or electronic parts, the necessity of efficiently radiating heat generated during the working of these parts to stabilize the parts by lowering the working temperature thereof and prevent the parts from being thermally deteriorated, has been more and more increased. For example, frequently a radiating fin made of a metal is fixed to a part radiating a large amount of heat, such as a power transistor and a diode, and in the case of assembling them, it has been the practice to interpose a radiating sheet comprising a rubber sheet having an electrically insulating property and also a good heat conductivity between them in order to efficiently perform the heat conduction. By fixing such a radiating sheet by applying a proper pressure, air existing between the surfaces of solids is excluded, whereby the heat conduction can be surely attained. When such a radiating sheet is not interposed, the heat conductivity of the electric parts or electronic parts themselves becomes an important characteristic.
Hitherto, a silicone rubber composition which becomes a rubber elastomer by being cured is well known and the silicone rubber composition has been widely used as potting materials for electric and electronic parts, coating materials, molding materials for mold forming, materials for electric wires, etc., by utilizing its excellent properties such as a weather resistance, a heat resistance, a cold resistance, an electrically insulating property, etc. Further, it is general practice to use the silcone rubber composition rendered heat conductive by compounding the silicone rubber composition with various heat-conductive materials. Alumina, aluminum nitride, boron nitride, ground qualtz, etc., are known as the heat-conductive material, but since most of the heat-conductive materials have a high Mohs' hardness, there are problems that a metal surface of a kneader (e.g., a Banbury mixer and a roll mixer) used at compounding the heat-conductive material is injured by the heat-conductive material, and when the heat-conductive material is mixed in large amounts with the silicone rubber composition, the rubber elasticity of the silicone rubber composition is lost, or the heat resistance of the composition is lowered.
Magnesium oxide is also used as the heat-conductive material and since the Mohs' hardness of magnesium oxide is 6, which is lower than those of the above-described other heat-conductive materials, the problems described above at compounding it with the silicone rubber composition do not occur. However, magnesium oxide has the substantial defect that magnesium oxide is hydrated with water or moisture with the passage of time and converted into magnesium hydroxide. Accordingly, when magnesium oxide is compounded with the silicone rubber composition as a heat-conductive material, the physical properties and the electric characteristics of the composition deteriorate, and thus it is difficult to say that magnesium oxide can be used in such system without feat of such deterioration occurring.
To avoid such problem, a method of burning magnesium oxide at a temperature of from 1,600.degree. C. to less than the melting point thereof is proposed as described in JP-A-61-85474 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, the method has the problem that single crystals of magnesium oxide are broken, various lattice defects occur on the surfaces of the crystals by the action of water or moisture, the form thereof becomes amorphous, and it is impossible to compound large amounts of such magnesium oxide with a silicone rubber composition.
Thus, a silicone rubber composition having a high heat conductivity is not obtained by the conventional techniques.